This invention relates to a vertical motor, a method of manufacturing this vertical motor, a polygon mirror motor using the vertical motor, and a bearing device for use in the vertical motor. The polygon mirror motor constitutes part of a beam scanning system, and is designed to prevent the tilting (i.e., fluctuation) of a surface of the polygon mirror. The vertical motor is designed to achieve a high-speed and high-precision operation of a laser recording device.
Various improvements in recording devices have heretofore been made, and in this connection various improvements in polygon mirror motors have conventionally been made.
As one example of such polygon mirror motors, Japanese Utility Model Examined Publication No. 63-1287 discloses a technique for preventing a mirror surface of a polygon mirror from being soiled with oil droplets scattered from a bearing portion during the rotation of the motor. Namely, when the mirror surface of the polygon mirror becomes soiled or dirty, its reflectance is lowered, and the power of the beam scanning over an object to be scanned (e.g. a photosensitive member) is lowered, which results in a disadvantage that an image is subjected to density uneveness. According to the technique disclosed in the above Japanese Utility Model Examined Publication No. 63-1287, the soiling of the mirror surface of the polygon mirror with the oil droplets scattered from the bearing portion during the rotation of the motor can be prevented.
However, in the conventional polygon mirror motors represented by the one disclosed in the above Japanese Utility Model Examined Publication No. 63-1287, a support member supporting the polygon mirror is held in contact with a lower surface of the polygon mirror over an entire upper surface thereof, and therefore there is much possibility that dust on the order of micron meter is caught in a gap between the entire upper surface of the support member and the lower surface of the polygon mirror. When the polygon mirror motor is assembled with dust (though on the order of micron meter) thus caught in the gap between the polygon mirror and the polygon mirror support member, there is a greater possibility that the surface of the polygon mirror is tilted.
If such surface tilting of the polygon mirror occurs, the position of reflected beam with respect to the object (e.g. a photosensitive material) to be scanned differs from one mirror surface to another to produce pitch uneveness in the image, when the beam emitted from the laser device is reflected by the mirror surface of the polygon mirror, as is well known in the art.
Further, the above Japanese Utility Model Examined Publication No. 63-1287 describes in FIG. 4 a ball bearing device. More specifically, a rotation sensing device is mounted on one end (free end) of a shaft, and a support member comprising a magnet is mounted around the other end of the shaft, and the polygon mirror is mounted on the support member. A drive coil is disposed outwardly of the bearing device to form an outer-rotor type polygon mirror motor having a magnet disposed outwardly of the drive coil.
Another known polygon mirror motor as shown in FIG. 12 is of the sealed type suitable for a highly-clear image processing. In a laser blink, a laser beam from a laser unit comprising a semiconductor laser or a gas laser is reflected by a mirror b of a rotating polygon rotor a to be applied to a back surface of a photosensitive member. The polygon rotor a is rotated by a drive motor c about a fixed shaft d through a sleeve e. A construction similar to this construction is disclosed in Japanese Patent Unexamined Publication No. 1-105015.
A number of dynamic pressure-generating grooves are formed in the outer peripheral surface of the fixed shaft d so as to produce dynamic pressure for bearing a thrust load and a radial load through the rotation of the rotary sleeve e. More specifically, with respect to the function of these dynamic pressure-generating grooves, the dynamic pressure is produced by herringbone-shaped lower grooves f1, intermediate grooves f2 and upper grooves f3 cooperating with the intermediate grooves f2 to provide a herringbone configuration, thereby bearing or supporting the radial load, and also air is fed to the upper surface of the fixed shaft d by the intermediate grooves f2 so as to increase the pressure of the air between the fixed shaft d and a thrust bearing g provided at the upper end of the fixed shaft d, thereby bearing the thrust load.
The polygon rotor a is fixedly mounted on the upper portion of the rotary sleeve e by screws, and a rotary magnet c1 is fixedly mounted on the lower portion of the rotary sleeve, and a stator coil c2 for driving the rotary magnet c1 is fixed in surrounding relation to the rotary magnet c1, thereby constituting the drive motor c. A laser incident window h is formed through an upper portion of a peripheral wall of an outer tube i, this window h allowing the passage of the laser beam to be applied from the exterior to the mirror b of the polygon rotor a, and also allowing the passage of the reflected laser beam to be applied to a desired afterglow surface. Since the polygon rotor a adapted to be rotated at high speeds by the drive motor c is required not only to maintain a high precision of its rotation but also to keep the displacement of the reflecting surface to a minimum, the gap between the fixed shaft d and the rotary sleeve e is quite narrow.
Further, Japanese Patent Unexamined Publication No. 1-105015 discloses a bearing device for use in a high-speed, vertical motor to control the whirl. This publication describes an air bearing having spiral bevel grooves.
The above-mentioned prior art is not intended to achieve a high-speed, high-precision design of the laser recording device, and also has the following problems:
First, since the above conventional motors are outer rotor motors, the inertia of the rotating part is large, and a rise time from the start to a rated rotational speed is long, and it takes long time before a fast recording. Therefore, they are not suited for a high-speed device.
Also, if a ball bearing is used as the bearing, the noise level increases with increase of speed, and a bearing loss occurs due to variations in fretting torque during the rotation of the balls and also due to deterioration of grease. Further, uneveness in rotation occurs due to displacement of the grease, and as the speed becomes higher, the follow-up of control circuitry becomes poorer, thus failing to control the uneveness in rotation of the motor. This results in a problem that a dislocation of vertical line recording is encountered.
These problems can be overcome by the use of an air bearing (as disclosed in the above conventional construction) free from variation in a bearing loss during the operation. However, such a construction is expensive and therefore is not suited for mass-production devices.
Further, since the start frequency lifetime of the air bearing is short, the polygon mirror motor must be kept in a standby condition by a continuous operation thereof. Therefore, the windage loss of the polygon motor as well as the core loss of the iron core increases in proportion to the square of the revolution number. This results in a problem that the operation cost in the standby condition abruptly increases in the case of the high-speed type polygon mirror motor.
Further, the air bearing device has, in addition to the above problems, a problem that its construction is considerably complicated.